package luogu.editor;

import java.util.*;

class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int input = Integer.parseInt(sc.nextLine());
        if(input>=90){
            System.out.println("4.0");
        } else if(input<90&&input>=60){
            int diff = 90 - input;
            System.out.println(String.format("%.1f", 4.0-0.1*diff));
        } else if(input<60){
            int score = (int)(Math.sqrt(input)*10);
            if(score<60){
                System.out.println("0.0");
            } else {
                int diff = 90 - score;
                System.out.println(String.format("%.1f", 4.0-0.1*diff));
            }
        }
    }
}